This invention relates to dynamic hue control networks. More particularly, this invention relates to dynamic hue control networks that are adapted to automatically vary hue dependent upon the level of the luminance component of the composite television signal.
By government regulation in North America a composite colour television signal comprises a luminance component, a chrominance component and a synchronising reference burst component. The luminance component is amplitude modulated on a carrier and contains information relating to the brightness of the image that is being telecast. The chrominance information is carried on a 3.58 MHz sub-carrier. This sub-carrier is amplitude modulated in respect of the degree of colour saturation and phase modulated in accordance with the tint of the colour. The colour sync bursts are transmitted only during the blanking intervals for the horizontal scan lines and provides a means whereby the reference signal can be reconstructed at the receiver.
The function of the sync bursts is to provide reference phase information. The locally generated reference signal must be in phase with the transmitted sub-carrier if the true hue of the telecast image is to be reproduced. Phase delay and distortion of colour signals during their transmission occur frequently and, because of this, it is common practice to provide a customer operated hue control that enables the viewer to adjust the phase of the chrominance signal relative to the locally generated reference signal, or vice versa, to obtain true colour reproduction.
The problem of hue errors is particularly acute at low luminance levels. When the luminance level is low, even if the hue control is adjusted so that the high luminance flesh colours are acceptably correct, it often will be found that the dark areas are incorrect, being surrounded by green tinges.
As the angle between the chroma demodulators departs from quadrature, and specifically for angles greater than quadrature, where the greens are significantly reduced, the larger demodulation angles produce considerably more acceptable colour reproduction. This effect has been used in various automatic tint control circuits. For example, reference may be made to U.S. Pat. No. 3,562,410, issued Feb. 9, 1971. The disadvantage of these automatic tint control circuits is that colour reproduction is limited to the orange-cyan axis, which, although producing acceptable flesh tones, severely limits the reproduction of other colours, particularly greens which become cyan, and purples which become red.
It has been shown by N. W. Parker in his paper entitled "An Analysis of the Necessary Decoder Corrections for Colour Receiver Operation with Non-Standard Receiver Primaries" (IEEE Transactions-Broadcast and Television Receivers -- volume 12, April 1966) that for the phosphors used in present day picture tubes, a demodulation angle of 105.degree. is about optimum. In practice, transmission standards and cable T.V. practices do not permit the use of angles much less than 120.degree. if objectionable greenish areas are to be avoided.